In modern cellular telephone systems, the transmission and reception of data has become almost as important as the transmission and reception of voice. Voice communications, in many newer networks, is carried by time division multiplexed access (TDMA), however, it is inefficient to use TDMA voice channels for data, since data does not place the same demands on the system for quality of service in terms of latency. Cellular digital packet data (CDPD) was developed to handle traffic originating in internet protocol (IP) format on cellular voice networks. CDPD is specified in the CDPD System Specification Release 1.1, 1998, published by The Wireless Data Forum.
When CDPD is used with TDMA, each cell in the cellular network has channels that can be used for CDPD data. In some systems, a channel is specifically dedicated to CDPD use. In other systems, one or more unused voice channels are used for CDPD when needed. Packet streams from multiple users are combined over the same channel, providing far more efficiency than would be possible if data was transmitted over voice channels. CDPD also conforms to the 7-layer, industry standard, open systems interconnect (OSI) model making it compatible with all major public and private packet switching networks. Despite these capabilities, however, the popularity of the Internet and the World Wide Web has put increasing pressure on CDPD resources distributed throughout the network.
The easiest way to increase the capacity of the CDPD network is to add channels to the cells. In high-traffic areas, a cell might have two, three, four, or even more CDPD channels for traffic from multiple users, either dedicated CDPD channels, or unused voice channels. However, full capacity of these channels is seldom realized. When a new transceiver is added to the CDPD load of the cell, it tries to register with the radio frequency (RF) channel with the highest signal strength. Typically, due to terrain and limitations on how transmitting towers are distributed within a cell, certain channels will be stronger in parts of the cell than other channels. In some cases, the network capacity does not increase significantly by just adding channels. Some channels bear most of the traffic while others remain underused.